1. Field of the Invention
The present invention relates to a lamp (e.g., discharge tube) lighting circuit.
2. Description of the Related Art
Heretofore, discharge tubes such as cold cathode fluorescent tubes have been frequently employed for the backlight of liquid crystal display devices, for example. FIG. 1 illustrates an example of a basic lighting circuit for a discharge tube. The lighting circuit shown in FIG. 1 includes an inverter V1001, a transformer T1001, and a discharge tube LP1001. The inverter V1001 is connected to terminals P1 and P2 of the primary winding of the transformer T1001, and one end of the discharge tube LP1001 is connected to a terminal S1 of the secondary winding of the transformer T1001. The other end of the discharge tube LP1001 and a terminal S2 of the secondary winding of the transformer T1001 are grounded. Boosting the output voltage of the inverter V1001 at the transformer T1001 lights the discharge tube.
Also, a lighting circuit according to another conventional technique is shown in FIG. 2. The lighting circuit shown in FIG. 2 includes an inverter V1002, transformers T1002 and T1003, and a discharge tube LP1002. The inverter V1002 is connected to terminals P1 and P2 of the primary winding of the transformer T1002, and terminals P1 and P2 of the primary winding of the transformer T1003. That is to say, the transformers T1002 and T1003 are connected to the inverter V1002 in parallel. Also, one end of the discharge tube LP1002 is connected to a terminal S1 of the secondary winding of the transformer T1002, and the other end of the discharge tube LP1002 is connected to a terminal S2 of the secondary winding of the transformer T1003. Note that the terminal S2 of the secondary winding of the transformer T1002 and the terminal S1 of the secondary winding of the transformer T1003 are grounded. That is to say, the left and right terminals of the discharge tube LP1002 are connected such that a reverse polarity voltage is applied thereto, thereby differentially driving the discharge tube LP1002. Accordingly, the leakage current as to stray capacitance is small, and also has a reversed phase, so becomes 0 in total, and a stable current flows, and accordingly, the luminance difference between the left and right of the discharge tube LP1002 is eliminated. However, in the event of employing two or more discharge tubes, the current between the discharge tubes cannot be made uniform without any change, resulting in increase of the number of inverters as well.
In recent years, around several through twenty discharge tubes have been employed for a single backlight due to the increased screen size of liquid crystal display devices and the like. At this time, the discharge tubes such as cold cathode fluorescent tubes have negative resistance properties wherein, upon current flowing, the voltage thereof suddenly drops, and increasing current causes the impedance thereof to gradually drop. Also, each discharge tube has an individual irregularity in impedance. These factors cause a problem wherein it is difficult to realize stable lighting and emission of each discharge tube. Accordingly, the following circuits have been employed.
FIG. 3 illustrates a lighting circuit according to a conventional technique for lighting a plurality of discharge tubes. The lighting circuit shown in FIG. 3 is a circuit for lighting four discharge tubes LP1004 through LP1007, and comprises a first circuit including an inverter V1004, a transformer T1004, a discharge tube LP1004, a resistance R1004, and a current detecting feedback line 1004, a second circuit including an inverter V1005, a transformer T1005, a discharge tube LP1005, a resistance R1005, and a current detecting feedback line 1005, a third circuit including an inverter V1006, a transformer T1006, a discharge tube LP1006, a resistance R1006, and a current detecting feedback line 1006, and a fourth circuit including an inverter V1007, a transformer T1007, a discharge tube LP1007, a resistance R1007, and a current detecting feedback line 1007. The inverter V1004 is connected to terminals P1 and P2 of the primary winding of the transformer T1004, one end of the discharge tube LP1004 is connected to a terminal S1 of the secondary winding of the transformer T1004, and the other end of the discharge tube LP1004 is connected to one end of the current detecting resistance R1004 and one end of the current detecting feedback line 1004. The other end of the resistance R1004 and a terminal S2 of the secondary winding of the transformer T1004 are grounded. The other end of the current detecting feedback line 1004 is connected to the inverter V1004. Hereafter, the connection relations regarding the second through fourth circuits are also the same as the first circuit, so description thereof will be omitted. With this lighting circuit, lighting of each discharge tube is controlled by controlling the inverter thereof according to current detected at the current detecting feedback line thereof. Thus, all of the discharge tubes are lit in a sure manner, whereby the current of all the discharge tubes can be made uniform.
FIG. 4 illustrates a lighting circuit according to another conventional technique. The lighting circuit shown in FIG. 4 is a circuit for lighting four discharge tubes LP1008 through LP1011, and includes an inverter V1008, transformers T1008 through T1011, capacitors C1008 through C1011, and discharge tubes LP1008 through LP011. The inverter V1008 is connected to terminals P1 and P2 of the primary winding of the transformer T1008; terminals P1 and P2 of the primary winding of the transformer T1009, terminals P1 and P2 of the primary winding of the transformer T1010, and terminals P1 and P2 of the primary winding of the transformer T1011. That is to say, the transformers T1008 through T1011 are connected to the inverter V1008 in parallel. Also, one end of the discharge tube LP1008 is connected to a terminal S1 of the secondary winding of the transformer T1008 via the capacitor C1008. That is to say, the discharge tube LP1008 and the capacitor C1008 are connected in series. Also, the other end of the discharge tube LP1008 and a terminal S2 of the secondary winding of the transformer T1008 are grounded. Further, one end of the discharge tube LP1009 is connected to a terminal S1 of the secondary winding of the transformer T1009 via the capacitor C1009. That is to say, the discharge tube LP1009 and the capacitor C1009 are connected in series. Also, the other end of the discharge tube LP1009 and a terminal S2 of the secondary winding of the transformer T1009 are grounded. One end of the discharge tube LP1010 is connected to a terminal S1 of the secondary winding of the transformer T1010 via the capacitor C1010. That is to say, the discharge tube LP1010 and the capacitor C1010 are connected in series. Also, the other end of the discharge tube LP1010 and a terminal S2 of the secondary winding of the transformer T1010 are grounded. One end of the discharge tube LP101 is connected to a terminal S1 of the secondary winding of the transformer T1011 via the capacitor C1011. That is to say, the discharge tube LP1011 and the capacitor C1011 are connected in series. Also, the other end of the discharge tube LP1011 and a terminal S2 of the secondary winding of the transformer T1011 are grounded. Thus, irregularities regarding current are suppressed by connecting a plurality of discharge tubes as to a single inverter in parallel, and inserting ballast condensers as to the respective discharge tubes in series for stable lighting and uniformity. Note that coils may be inserted instead of ballast condensers, or transformers also serving as ballast coils, which generate leakage inductance by intentionally deteriorating a coupling coefficient, may be provided as to the respective discharge tubes.
FIG. 5 illustrates a lighting circuit according to yet another conventional technique. The lighting circuit shown in FIG. 5 includes an inverter V1012, transformers T1012 through T0116, and discharge tubes LP1012 through LP1015. The inverter V1012 is connected to terminals P1 and P2 of the primary winding of the transformer T1012, and terminals P1 and P2 of the primary winding of the transformer T1016. That is to say, the transformers T1012 and T1016 are connected to the inverter V1012 in parallel, which realizes differential driving. Also, a terminal S1 of the secondary winding of the transformer T1012 is connected to a terminal P2 of the primary winding and a terminal S1 of the second winding of the transformer T1013. Also, a terminal P1 of the primary winding of the transformer T1013 is connected to one ends of the discharge tubes LP1014 and LP1015. Further, a terminal S2 of the secondary winding of the transformer T1013 is connected to one of the ends of the discharge tubes LP1012 and LP1013. On the other hand, the other end of the discharge tube LP1015 is connected to a terminal P1 of the primary winding of the transformer T11015, and the other end of the discharge tube LP1014 is connected to a terminal S2 of the secondary winding of the transformer T1015. A terminal P2 of the primary winding and a terminal S1 of the secondary winding of the transformer T1015 are connected to a terminal S2 of the secondary winding of the transformer T1016 and a terminal P1 of the primary winding and a terminal S2 of the secondary winding of the transformer T1014. Also, the other end of the discharge tube LP1012 is connected to a terminal S1 of the secondary winding of the transformer T1014, and the other end of the discharge tube LP1013 is connected to a terminal P2 of the primary winding of the transformer T1014. A terminal S1 of the secondary winding of the transformer T1016 and a terminal S2 of the secondary winding of the transformer T1012 are grounded. Thus, with this lighting circuit, three common mode chokes, i.e., one-on-one transformers (transformers T1013 through T1015) are employed for the four discharge tubes LP1012 through LP1015, thereby realizing stable lighting and uniformity.
FIG. 6 illustrates a lighting circuit according to yet another conventional technique. The lighting circuit shown in FIG. 6 includes an inverter V1017, transformers T1017 through T1020, and discharge tubes LP1017 through LP1020. The inverter V1017 is connected to a terminal P1 of the primary winding of the transformer T1020, and a terminal P2 of the primary winding of the transformer T1017. Also, a terminal P2 of the primary winding of the transformer T1020 and a terminal P1 of the primary winding of the transformer T1019 are connected, a terminal P2 of the primary winding of the transformer T1019 and a terminal P1 of the primary winding of the transformer T1018 are connected, and a terminal P2 of the primary winding of the transformer T1018 and a terminal P1 of the primary winding of the transformer T1017 are connected. That is to say, the transformers T1017 through T1020 and the inverter V1017 are connected in series. Further, a terminal S2 of the secondary winding of the transformer T1017 is connected to one end of the discharge tube LP1017, a terminal S2 of the secondary winding of the transformer T1018 is connected to one end of the discharge tube LP1018, a terminal S2 of the secondary winding of the transformer T1019 is connected to one end of the discharge tube LP1019, and a terminal S2 of the secondary winding of the transformer T1020 is connected to one end of the discharge tube LP1020. The other ends of the discharge tubes LP1017 through LP1020 and terminals S1 of the secondary windings of the transformers T1017 through T1020 are grounded. Thus, the secondary windings of the transformers and the discharge tubes are connected independently, thereby essentially yielding the same advantage as when connecting discharge tubes themselves to the inverter in series.
FIG. 7 illustrates a lighting circuit according to a conventional technique for lighting a plurality of discharge tubes. The lighting circuit shown in FIG. 7 is a circuit for lighting four discharge tubes LP1021 through LP1024, and comprises a first circuit including an inverter V1021, transformers T1021 and T1025, a discharge tube LP1021, a resistance R1021, and a current detecting feedback line 1021, a second circuit including an inverter V1022, transformers T1022 and T1026, a discharge tube LP1022, a resistance R1022, and a current detecting feedback line 1022, a third circuit including an inverter V1023, transformers T1023 and T1027, a discharge tube LP1023, a resistance R1023, and a current detecting feedback line 1023, and a fourth circuit including an inverter V1024, transformers T1024 and T1028, a discharge tube LP1024, a resistance R1024, and a current detecting feedback line 1024. The inverter V1021 is connected to terminals P1 and P2 of the primary winding of the transformer T1021, and terminals P1 and P2 of the primary winding of the transformer T1025. However, the inverter V1021 and the transformer T1021, and the inverter V1021 and the transformer T1025 are connected so as to generate a reversed phase. A terminal S1 of the secondary winding of the transformer T1021 is connected to a first terminal of the discharge tube LP1021, and a second terminal of the discharge tube LP1021 is connected to a terminal S1 of the secondary winding of the of the transformer T1025. A terminal S2 of the secondary winding of the transformer T1021 is grounded via the resistance R1021, and a terminal S2 of the secondary winding of the transformer T1025 is directly grounded. Also, the current detecting feedback line 1021 is connected to a terminal S2 of the secondary winding of the transformer T1021 and the inverter V1021. Hereafter, the connection relations regarding the second through fourth circuits are also the same as the first circuit, so description thereof will be omitted. With this lighting circuit, lighting of each discharge tube is controlled by controlling the inverter thereof according to current detected at the current detecting feedback line thereof. Thus, all of the discharge tubes are lit in a sure manner, whereby the current of all the discharge tubes can be made uniform. However, this technique requires many more inverters, thereby leading prohibitively high costs.
FIG. 8 illustrates a lighting circuit according to another conventional technique. The lighting circuit shown in FIG. 8 is a circuit for lighting four discharge tubes LP1025 through LP1028, and includes an inverter V1025, transformers T1029 through T1036, ballast condensers C1025 through C1028, and discharge tubes LP1025 through LP1028. The inverter V1025 is connected to terminals P1 and P2 of the primary winding of the transformer T1029, terminals P1 and P2 of the primary winding of the transformer T1030, terminals P1 and P2 of the primary winding of the transformer T1031, and terminals P1 and P2 of the primary winding of the transformer T1032. Also, the inverter V1025 is, so as to generate the reversed phase as to the phase of the above transformers, connected to terminals P1 and P2 of the primary winding of the transformer T1033, terminals P1 and P2 of the primary winding of the transformer T1034, terminals P1 and P2 of the primary winding of the transformer T1035, and terminals P1 and P2 of the primary winding of the transformer T1036. A terminal S2 of the secondary winding of the transformer T1029 is connected to a first terminal of the discharge tube LP1025 via the ballast condenser C1025, and a second terminal of the discharge tube LP1025 is connected to a terminal S2 of the secondary winding of the transformer T1033. A terminal S2 of the secondary winding of the transformer T1030 is connected to a first terminal of the discharge tube LP1026 via the ballast condenser C1026, and a second terminal of the discharge tube LP1026 is connected to a terminal S2 of the secondary winding of the transformer T1034. A terminal S2 of the secondary winding of the transformer T1031 is connected to a first terminal of the discharge tube LP1027 via the ballast condenser C1027, and a second terminal of the discharge tube LP1027 is connected to a terminal S2 of the secondary winding of the transformer T1035. A terminal S2 of the secondary winding of the transformer T1032 is connected to a first terminal of the discharge tube LP1028 via the ballast condenser C1028, and a second terminal of the discharge tube LP1028 is connected to a terminal S2 of the secondary winding of the transformer T1036. Note that the residual terminals S1 of the secondary windings of the transformers T1029 through T1036 are grounded. Employing such a lighting circuit can reduce the number of inverters, but only the irregularities of discharge tube impedance are alleviated by ballast condenser impedance, so current cannot be sufficiently made uniform. Also, a high-voltage capacity power is necessary for maintaining a high voltage even when a large current flows into a lit discharge tube to eliminate partial non-lighting of discharge tubes on startup, which deteriorates electrocution safety.
FIG. 9 illustrates a lighting circuit disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 59-187097. The lighting circuit shown in FIG. 9 includes an inverter V1027, transformers T1039 and T1040, discharge tubes LP1031 and LP1032, capacitors C1029 through C1031. The inverter V1027 is connected to terminals P1 and P2 of the primary winding of the transformer T1039, and terminals P1 and P2 of the primary winding of the transformer T1040. Note that a terminal P1 of the primary winding and a terminal S1 of the secondary winding of the transformer T1039 are connected via the capacitor C1030, and a terminal P1 of the primary winding and a terminal S1 of the secondary winding of the transformer T1040 are also connected via the capacitor C1031. Further, a first terminal of the discharge tube LP1031 is connected to a terminal S1 of the secondary winding of the transformer T1039, and a second terminal of the discharge tube LP1039 is connected to a terminal S1 of the secondary winding of the transformer T1040. A first terminal of the discharge tube LP1032 is connected to a terminal S2 of the secondary winding of the transformer T1039, and a second terminal of the discharge tube LP1032 is connected to a terminal S2 of the secondary winding of the transformer T1040 via the capacitor C1029. Thus, the discharge tubes LP1031 and LP1032 are configured such that the left and right bipolarities are driven with anti-polarity floating differential driving. However, the secondary windings of the transformers T1039 and T1040 are connected to the capacitors, which provides a problem wherein current balance between both the discharge tubes LP1031 and LP1032 is shifted by only the amount of capacities of these capacitors, resulting in difference between luminance of the discharge tubes. Also, consideration is not made regarding whether or not what kind of configuration is preferable in the event of employing three or more discharge tubes.
Note that though not shown in the drawing, with Japanese Unexamined Patent Application Publication No. 61-195592, a discharge tube lighting device comprising an AC power source, a series circuit made up of a plurality of discharge lamps connected to this AC power source, sequence impedance connected to at least one of the plurality of discharge lamps in parallel, a preheating transformer of which the primary winding is connected to the AC power source, and the secondary winding is connected to each filament of the plurality of discharge lamps, a prior preheating switch subjected to through-insertion connection between the AC power source and the primary winding of the preheating transformer, and a short switch, which is connected to the primary winding of the preheating transformer in parallel, and turned on following the plurality of discharge lamps being turned on. However, this publication aims at preventing the preheating transformer from heat generation due to iron losses when the lighted discharge lamp connected to the sequence impedance in parallel is removed, which includes some description for attempting sharing of the transformer, but degree of reduction thereof is insufficient. Also, the configuration at the time of lighting the three or more discharge lamps is not necessarily cleared.
Further, as for the simplest method for subjecting current which flows into a discharge tube to uniformity, there is a method for connecting a plurality of discharge tubes to the secondary winding of a transformer in series, but voltage for the worth of the number of discharge tubes is accumulated, and accordingly, resulting in extremely high voltage, so extreme withstanding high-power is required for the transformer and wiring. Moreover, danger of electrocution increases. Further, lighting using a high-frequency inverter causes a problem wherein brightness is not made uniform since current made to flow differs according to the position of a discharge tube due to the influence of a leakage current made to flow into stray capacitance such as an electroconductive backplane and the like from a discharge tube or a lead wire.
See Japanese Unexamined Patent Application Publication No. 9-237686, Japanese Examined Utility Model Registration Application Publication No. 64-005360, Japanese Unexamined Utility Model Registration Application Publication No. 59-187097, and Japanese Unexamined Patent Application Publication No. 61-195592, regarding the conventional art described here.
In the case of the lighting circuit such as shown in FIG. 3, current made to flow into each discharge tube is independently controlled, thereby achieving stable lighting and uniformity with ease, but this needs to provide an expensive inverter for each discharge tube, so as a whole, resulting in too expensive costs.
Also, in the case of the lighting circuit such as shown in FIG. 4, the lighting circuit can be configured at low costs, but this simply alleviates the irregularities of the total impedance as to the discharge tubes by series impedance, so ballast effects are limited, and the irregularities of loading for each discharge tube cannot be sufficiently absorbed, and accordingly, uniformity is limited. Also, the voltage of previously lighted discharge tubes deteriorates at the time of lighting, which attempts to prevent the other unlighted discharge tubes from lighting, so it is necessary to provide a powerful inverter circuit so as to prevent deterioration of the voltage, and accordingly, a great current is necessary at the moment of start-up. Also, the output voltage of the inverter needs to be increased for the worth of ballast, resulting in increase of power loss.
Further, in the case of the lighting circuit such as shown in FIG. 5, the light circuit can be configured at low cost, and uniformity can be achieved as well, but the inverter has a large-current driving capability appropriate for high-voltage output and the number of the discharge tubes, which is very dangerous when short-circuiting electrocution.
Also, the lighting circuit such as shown in FIG. 6 has various excellent features, but as liquid crystal display devices increase in size and finer definition, the number of cold cathode fluorescent tubes to be employed as a backlight increases, and realizing a circuit having also more uniform brightness and less noise than the circuit such as shown in FIG. 6 is demanded.
Further, as described above, in the event of lighting three or more discharge tubes, the conventional lighting circuits cause various problems such as increase of the number of transformers, problems regarding reliability and safety of lighting, difficulty in uniformity of current made to flow into each discharge tube, and the like.